Gliederung

Background: Heavy ion therapy offers a dose conformity superior to any other external irradiation technique, thanks to its advantageous depth dose profile and increased biological effectiveness. However, the density patterns along the ray paths have a strong influence on the Bragg peak position, rendering the technique sensitive to patient/organ mispositioning. Unlike the case of photon irradiation, addition of a safety margin around the target is often an insufficient countermeasure. A possible solution is instead to optimize robustness at planning time, e.g. by avoiding significant density interfaces in the field. The goal of this study was to introduce and verify an index scoring the robustness of the beam directions and thus assisting the optimum beam setup choice.

Material and methods: To numerically quantify the robustness of a beam setup a Port Homogeneity Index (PHI) was defined which scores the influence of the density patterns along a beam track by means of information theoretic measures. The index was used to support the choice of beam directions in planning 10 patients with base of the skull tumors in regions where density heterogeneities were critical. For each patient two clinically plausible beam setups were chosen: one with low PHI and one with high PHI.

For each patient and for each beam setup a carbon ion plan was calculated with the raster scanning technique using the TRiP software in single field optimization mode. A planning margin of 2 mm was applied.

Subsequently, rigid positioning errors of 1–2 mm in AP, SI and LR directions were simulated by introducing a shift of the irradiation fields with respect to the CT data and recomputing the plans without raster re-optimization. Consequences on the dose conformity were assessed by means of dose distribution display, DVH analysis and calculation of some commonly used dosimetric quality indexes.

Results: The initial plans showed comparable quality of target coverage for both beam setups. However, the plans prepared for beam configurations with low Port Homogeneity Index were often characterized by lack of robustness against mispositioning, as demonstrated by the effects of the simulated shifts on the dosimetry of such plans: cold spots down to 90% of the prescribed dose, relatively stronger deterioration of the DVHs and high variability of the dosimetric quality indexes as functions of the positioning error. On the contrary plans where a high PHI was chosen, and regions of extreme density interfaces avoided, proved to be less sensitive to repositioning errors.

Conclusion: The Port Homogeneity Index proved to be an effective tool for the choice of beam directions. It offers substantial support in avoiding severe density interfaces, therefore significantly improving the robustness of plans against residual positioning errors, which is of the utmost importance for heavy ion therapy, in particular if multiple field optimization and intensity modulation (IMPT) are used. Figure 1 [Fig.Â 1].